Chapter 21 (Alpha Carbon Chemistry)

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26 Terms

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Acid-catalyzed (ketone) tautomerization mechanism

  • Tautomers are rapidly interconverting constitutional isomers that differ double bond placement and proton placement

  • Tautomerization refers to the formation of a tautomer. 

    • In the case of a ketone, the tautomer that is formed is an enol

  • In acidic conditions, the ketone and enol are present at equilibrium

<ul><li><p><strong>Tautomers</strong> are rapidly interconverting constitutional isomers that differ double bond placement and proton placement</p></li><li><p><strong>Tautomerization</strong> refers to the formation of a tautomer.&nbsp;</p><ul><li><p>In the case of a ketone, the tautomer that is formed is an <strong>enol</strong></p></li></ul></li><li><p>In acidic conditions, the ketone and enol are present at equilibrium</p></li></ul><p></p>
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Base-catalyzed (ketone) tautomerization mechanism

  • In basic conditions, the ketone and enol are present in equilibrium

<ul><li><p>In basic conditions, the ketone and enol are present in equilibrium</p></li></ul><p></p>
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Enolate formation mechanism

  • An enolate is the resonance-stabilized intermediate that forms from treating ketone with a strong base

    • From from deprotonation at the alpha position

  • When the ketone treated with strong base, enol and enolate both present but enolate is predominant and more reactive

<ul><li><p>An <strong>enolate</strong> is the resonance-stabilized intermediate that forms from treating ketone with a strong base</p><ul><li><p>From from deprotonation at the alpha position</p></li></ul></li><li><p>When the ketone treated with strong base, enol and enolate both present but enolate is predominant and more reactive</p></li></ul><p></p>
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  1. [H3O+], Br2

(Ketone/aldehyde substrate)

  • Alpha halogenation

  • Halogenation occurs at the alpha position

<ul><li><p><strong><u>Alpha halogenation</u></strong></p></li><li><p>Halogenation occurs at the alpha position</p></li></ul><p></p>
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  1. Br2, PBr3

  2. H20 

(CA substrate)

  • Alpha halogenation of the CA

  • Hell-Volhard-Zelinsky (HVZ) reaction

  • Br added to alpha position of CA carbonyl

<ul><li><p><strong><u>Alpha halogenation</u></strong> of the CA</p></li><li><p>Hell-Volhard-Zelinsky (HVZ) reaction</p></li><li><p>Br added to alpha position of CA carbonyl</p></li></ul><p></p>
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  1. NaOH, Br2

(Cyclic ketone substrate)

  • Base-catalyzed alpha halogenation

  • Bromination occurs until all alpha protons are removed and replaced with Br, I, or Cl

<ul><li><p><strong><u>Base-catalyzed alpha halogenation</u></strong></p></li><li><p>Bromination occurs until all alpha protons are removed and replaced with Br, I, or Cl</p></li></ul><p></p>
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  1. NaOH, Br2

  2. H3O+

(Methyl ketone substrate)

  • Formation of a carboxylic acid + CHBr3 (bromoform)

<ul><li><p>Formation of a carboxylic acid + CHBr3 (bromoform) </p></li></ul><p></p><p></p>
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  • Aldol formation

  • Enolate and aldehyde present at equilibrium

  • Note: hydroxyl located beta relative to carbonyl

    • Product of aldol reaction is always a beta-hydroxy aldehyde or beta-hydroxy ketone

<ul><li><p><strong><u>Aldol formation</u></strong></p></li><li><p>Enolate and aldehyde present at equilibrium</p></li><li><p>Note: hydroxyl located beta relative to carbonyl</p><ul><li><p>Product of aldol reaction is always a beta-hydroxy aldehyde or beta-hydroxy ketone</p></li></ul></li></ul><p></p>
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Aldol addition mechanism

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  • Aldol condensation

    • Condensation = reaction where addition is accompanied by the loss of a small molecule (water, CO2, or N2)

    • Form alpha,beta-unsaturated ketone/aldehydes

<ul><li><p><strong><u>Aldol condensation</u></strong></p><ul><li><p>Condensation = reaction where addition is accompanied by the loss of a small molecule (water, CO2, or N2)</p></li><li><p>Form alpha,beta-unsaturated ketone/aldehydes</p></li></ul></li></ul><p></p>
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Aldol condensation mechanism

  • E1CB mechanism — elimination that involves the formation of an anion.

  • Trans configuration will be major 

<ul><li><p><strong>E1CB</strong> mechanism — elimination that involves the formation of an anion.</p></li><li><p>Trans configuration will be major&nbsp;</p></li></ul><p></p>
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  • Intramolecular aldol reaction

<ul><li><p>Intramolecular aldol reaction</p></li></ul><p></p>
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Intramolecular aldol reaction mechanism

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  • Claisen condensation reaction

<ul><li><p>Claisen condensation reaction</p></li></ul><p></p>
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Claisen condensation mechanism

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  • Intramolecular Claisen condensation

<ul><li><p>Intramolecular Claisen condensation</p></li></ul><p></p>
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  • Alkylation at the alpha position

<ul><li><p>Alkylation at the alpha position</p></li></ul><p></p>
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Alkylation mechanism

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  • Kinetic vs. thermodynamic product formation in alkylation at the alpha position

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  • Acetoacetic ester synthesis

    • Results in an alkylation product while preventing polyalkylation

    • Hydronium results in reduction of the ester to a carboxylic acid

    • Heat results in cleavage to loose the carboxylic acid group

<ul><li><p>Acetoacetic ester synthesis</p><ul><li><p>Results in an alkylation product while preventing polyalkylation</p></li><li><p>Hydronium results in reduction of the ester to a carboxylic acid</p></li><li><p>Heat results in cleavage to loose the carboxylic acid group</p></li></ul></li></ul><p></p><p></p>
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  • Malonic ester synthesis

    • Alkylation product that conserves a carboxylic acid group

<ul><li><p>Malonic ester synthesis</p><ul><li><p>Alkylation product that conserves a carboxylic acid group</p></li></ul></li></ul><p></p>
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  • Beta alkylation

    • Occurs with the treatment of an alpha,beta unsaturated ketone with lithium dialkyl cuprate

<ul><li><p>Beta alkylation</p><ul><li><p>Occurs with the treatment of an alpha,beta unsaturated ketone with lithium dialkyl cuprate </p></li></ul></li></ul><p></p>
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  • Alkylation at alpha and beta positions

<ul><li><p>Alkylation at alpha and beta positions</p></li></ul><p></p>
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  • Michael reaction

    • Michael donor — the highly stabilized enolate

    • Michael acceptor — alpha, beta unsaturated ketone

<ul><li><p>Michael reaction</p><ul><li><p>Michael donor — the highly stabilized enolate</p></li><li><p>Michael acceptor — alpha, beta unsaturated ketone</p></li></ul></li></ul><p></p>
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  • Stork enamine synthesis

  • Used when need to accomplish addition at the alpha position but do not have a highly stabilized enolate 

<ul><li><p>Stork enamine synthesis</p></li><li><p>Used when need to accomplish addition at the alpha position but do not have a highly stabilized enolate&nbsp;</p></li></ul><p></p>
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Stork enamine synthesis mechanism

  • Notice: beta position attacked

<ul><li><p><strong><u>Notice: beta position attacked</u></strong></p></li></ul><p></p>